The invention relates to methods and apparatus for thermally processing packaging containers, in particular (but not exclusively) metal cans containing a food product for human or animal consumption.
In a normal canning process the food product is filled into empty cans to an appropriate level, leaving a headspace above the product, the open ends of the cans are hermetically sealed with end closures, and then the cans and their contents are sterilised by means of heat. The heating medium used is normally either steam or hot water at a temperature usually of between 115.degree. C. and 130.degree. C. To achieve this temperature the steam or hot water has to be held at a superatmospheric pressure, and accordingly it is contained in a pressure vessel known as a retort or cooker.
The cans, after filling and closing, are placed in the retort, the retort is closed, and steam or water is introduced. Temperature controllers are usually present on the retort to maintain the heating medium at the desired temperature. While the cans are located in the retort, heat from the heating medium is conducted through the container walls and thence passes into the product.
Insofar as their behaviour during thermal sterilisation is concerned, food products are usually classified into three categories, namely: (1) those that heat largely by convection, (2) those that heat largely by conduction, and (3) those that heat by substantial parts each of conduction and convection. Food products having a very thin consistency heat largely by convection, that is to say, the heating process generates convection currents in the product and these currents disperse the heat throughout the pack; products of this kind fall into category 1. Thick, e.g. relatively viscous or partly particulate, products, heat largely by conduction; for them no significant movement within the container occurs, and so heat can substantially only be transferred by conduction; these products form category 2. The products falling within category 3, which heat by both conduction and convection, form the smallest of the three categories and include those products which either thicken or become substantially more fluid as heating progresses.
Because of the need for complete sterilisation all parts of the food product in a can must reach a sufficient temperature for a long enough time to achieve so-called commercial sterility. With non-acid (pH&gt;4.5) products which heat largely by convection (i.e. category 1 products) this occurs fairly quickly; for example, a cylindrical can of 73 mm diameter and 110 mm length typically takes 15-20 minutes in a retort at 121.degree. C. to heat to sterilisation temperature ("heat-up time") and remain at that temperature for as long as may be necessary to achieve commercial sterility ("dwell time"). The 15-20 minute period, thus made up of the heat-up time and any dwell time of the can in the retort at sterilisation temperature, is commonly referred to as the "process" time, which nomenclature will be used hereafter. The process time is subsequent to any time which may be allowed for the retort itself to heat to sterilisation temperature, hereinafter referred to as the "come-up" time. The come-up time may be considerable, e.g. up to half hour, and some heating of the cans will occur during this time.
The time period formed of the heat-up time of the cans and any come-up time of the retort is significant in the context of the present invention because it represents the time during which the product in the cans is being heated to the sterilisation temperature by heat passing through the can wall. This time period, hereinafter referred to as the "heating" time (of the cans), may be supplemented by any dwell time to form the total time during which the cans are subjected to the heating medium and which accordingly is hereinafter referred to as the "sterilisation", or more generally, "thermal treatment" time.
It will be seen that, using the definitions given above, the sterilisation time is equal to the process time plus any come-up time; it is also equal to the heating time plus any dwell time.
Category 2 products require much longer heating times than category 1 products because of their lesser mobility; a can as described above but charged with a category 2 product may typically have a process time of 80-90 minutes at 121.degree. C., and to this must be added any retort come-up time allowed and, in addition, the time required for the hot and sterile can to cool to a predetermined temperature at which it may safely be removed from the retort. This latter time duration is hereinafter referred to as the "cooling" time of the can. Thus the total time required by the complete sterilisation cycle, i.e. from admission of the heating medium to the end of cooling, may be 2 hours or more; this overall time is hereinafter called the "total cycle time".
The long heating times required by category 2 packs (in particular) often lead to overcooking of the product, especially where it lies adjacent to the container wall. In commercial practice it is already well known to reduce the heating time and possible overcooking of a category 2 food product in a static retort by agitating the can by rotating it whilst in the retort. The rotation of the can has been either about its cylindrical axis, or "end-over-end" about a transverse (diametral) axis through its centre. The first form of agitation can be generated by rolling cans of circular section about their longitudinal axis, and is used in "Reel" and "Spiral" cookers; however, it is well recognised that it does not induce efficient mixing, and the required process times are reduced by a factor of only about 2. `End-over-end` rotation induces better mixing, and reduction factors in process time of of 3 or 4 can be expected.
In addition to the commercially used methods described above there are proposals in the patent literature for achieving process time reductions by agitation. These proposals have variously employed vertical or horizontal reciprocation (i.e. back-and-forth movement along a substantially straight path), or angular movement, possibly with reversals, along a circular path, or compound movement having both reciprocating and angular components.
By way of example, vertical reciprocation is featured in U.S. Pat. No. 1,709,175 and German Patent Publication 2031822, whilst horizontal reciprocation is featured in U.S. Pat. Nos. 2,052,096 and 2,134,817, and Japanese Patent Publication JP 56-21584. Angular movement is featured in GB Patent Specification 1593962 at FIGS. 12, 13 and at FIGS. 14, 15, whilst compound movement is featured in FIGS. 16, 17 of GB 1593962 and in French Patent Publication 2096516.
Whilst these and other proposals in the patent literature might be expected to achieve useful reductions in process time with the attendant advantages, they contain no indication that the severity of the agitation is important and, moreover, the maximum acceleration given to the container must exceed a certain minimum value if the process is to be reliably reproducible whilst achieving high levels of process time reduction.
For example, in the process particularly described in U.S. Pat. No. 2,134,817 above, the amplitude of the horizontal reciprocating movement between limiting positions (i.e. the peak-to-peak or double amplitude) is said to be usually less than one inch, and the reciprocation frequency is said to be in the neighbourhood of 140 times (i.e. cycles) per minute.
Assuming a sinusoidal waveform for the reciprocation, these parameter values given for the process of U.S. Pat. No. 2,134,817 correspond to a maximum value of acceleration of approximately 0.3 times that due to gravity (i.e. 0.3 g). As is manifest, however, from the accompanying graphs showing the results of tests made by the present Applicants, horizontal reciprocation using accelerations of this magnitude stands only to achieve a reduction of heating times (in relation to the same process without reciprocation) which is little or no better than the reduction which is commonly achieved by the commercially practiced methods described above in which the cans are rotated either about their longitudinal axes or end-over-end. Moreover, and as will be discussed more fully later, our tests have indicated that the process described in U.S. Pat. No. 2,134,817 will be subject to wide random variations, and as a result the sterilisation times which would be required in practice to ensure commercial sterility using that process would have to be made considerably greater than the sterilisation times which can be achieved.
As mentioned above, in the proposal of U.S. Pat. No. 2,134,817 the cans are reciprocated horizontally. Comparative tests performed by the present Applicants have shown that horizontal reciprocation offers more efficient and more uniform product mixing of Category 2 food products than does vertical reciprocation, and for this and other reasons horizontal reciprocation is preferred. For some containers, for example, cylindrical cans which are longer than they are wide, it is advantageous for them to be generally aligned with the reciprocation path. For other containers, however,--for example, squat cylindrical cans--it may be preferred for them to be orientated differently in relation to the reciprocation path.
The present invention seeks to provide significant and consistent reductions in the heating time of a product in a container in a thermal treatment process, particularly (but not necessarily) a sterilisation process, and accordingly provides, in accordance with a first aspect thereof, a process for thermally treating a product in a container having a headspace above the product, in which the container is subjected to a heated environment and is simultaneously agitated, characterised in that the acceleration to which the container is subjected by the agitation is of sufficient magnitude to cause the process to operate in a regime in which the heating time required for the product to reach a predetermined temperature is very substantially reduced and moreover is substantially insensitive to changes in the acceleration. Preferably the acceleration is such that the heating time of the product is reduced by at least 90% and has a gradient of at most 1 min per g of acceleration.
The invention may also advantageously be used for reducing the cooling time of a product in a container from an elevated temperature to a predetermined lower temperature which may be near or equal to ambient temperature. In accordance with a second aspect thereof the invention accordingly provides a process for cooling a hot product in a container having a headspace above the product, characterised in that the container is subjected to a cooled environment and is simultaneously reciprocated with an acceleration of sufficient magnitude to cause the process to operate in a regime in which the cooling time required for the product to reach a predetermined temperature is very substantially reduced and moreover is substantially insensitive to changes in the acceleration. Preferably the acceleration is such that the cooling time of the product is reduced by at least 90% and has a gradient of at most 1 min per g of acceleration. Usually the product is thermally treated by a process as defined in the preceding paragraph, and, subsequent to the thermal treatment, is cooled to a lower temperature by the process forming this second aspect of the invention.
The word "headspace" as used above generally has the meaning which is conventional in the food canning industry, that is to say, it denotes the distance by which the surface of the product in the can falls short of the top free edge of the can at the end of the product filling operation but before the closure is seamed into position to close the can. In the remainder of the specification and claims this headspace is hereinafter referred to as the "gross headspace", in order to differentiate it from the actual headspace existing in the can after the end closure has been attached. The latter form of headspace, hereinafter referred to as "net headspace", is smaller than the gross headspace by about 3 mm. Thus, for example, a gross headspace of 12 mm may be considered to correspond to a net headspace of 9 mm, one of 8 mm to a net headspace of 5 mm, and one of 4 mm to a net headspace of 1 mm.